In recent years, fuel cells have been attracting attention as a next generation energy source. In particular, polymer electrolyte fuel cells (PEFCs), which use a proton-conductive polymer membrane as the electrolyte, have high energy density and are expected to find a wide range of applications, such as in home cogeneration system, power sources for mobile devices, and power sources for automobiles. The electrolyte membrane of a PEFC needs to function as an electrolyte for conducting protons between the fuel electrode and the oxidant electrode and to serve as a partition wall for separating a fuel supplied to the fuel electrode and an oxidizing agent supplied to the oxidant electrode. If one of the functions as the electrolyte and the partition wall is insufficient, the power generation efficiency of the fuel cell degrades. For this reason, a polymer electrolyte membrane is desired that is excellent in proton conductivity, electrochemical stability, and mechanical strength and has low permeability of fuel and oxidizing agent.
Currently, perfluorocarbon sulfonic acid having a sulfonic acid group as the proton conductive group (for example, Nafion (registered trademark) made by DuPont Corp.) is widely used for the electrolyte membrane of the PEFC. Although the perfluorocarbon sulfonic acid membrane shows excellent electrochemical stability, it is very costly because the fluororesin that is a source material is not a general-purpose product and also the process of synthesis is complicated. A high-cost electrolyte membrane can be a great barrier to commercialization of the PEFC. Moreover, the perfluorocarbon sulfonic acid membrane easily permeates methanol (i.e., it is poor in methanol blocking properties), therefore, it is difficult to use the perfluorocarbon sulfonic acid membrane as the electrolyte membrane of a direct methanol fuel cell (DMFC), one type of the PEFC, in which a methanol-containing solution is supplied to the fuel electrode.
For these reasons, development of a hydrocarbon polymer electrolyte membrane that is low in cost and inhibits the methanol permeation (cross-over) has been underway as a replacement of the perfluorocarbon sulfonic acid membrane. For example, JP 6(1994)-93114 A, JP 10(1998)-45913 A, JP 9 (1997)-245818 A, and JP 2000-510511 T (Published Japanese translation of PCT application) propose electrolyte membranes made of sulfonated poly(ether ether ketone), sulfonated poly(ether sulfone), sulfonated polysulfone, and sulfonated polyimide, respectively. The resins used as the source materials of these hydrocarbon electrolyte membranes are lower in cost than fluororesin, so the use of the above electrolyte membranes is claimed to achieve cost reduction of the PEFC. However, the characteristics of the hydrocarbon electrolyte membranes proposed in the just-mentioned publications are not necessarily sufficient for the electrolyte membrane for a fuel cell, which is required to have both functions of an electrolyte and a partition wall, and commercialization of a PEFC using the membranes has yet not been accomplished.
Apart from them, JP 2006-156041 A discloses an electrolyte membrane made of a blend of a water-soluble polymer electrolyte having an acid group, which is a proton conductive group, polyvinyl alcohol (PVA), and a water-soluble polymer such as polyethylene glycol (PEG), which is a third component (see claim 1). It also discloses that the membrane may be physically or chemically crosslinked (see claim 4, paragraph [0006], and the examples). The use of PVA as the base material allows this electrolyte membrane to be manufactured at low cost. However, the proton conductivity of the electrolyte membrane disclosed in JP 2006-156041 A is significantly poorer than that of the perfluorocarbon sulfonic acid membrane. It is impracticable to use the just-mentioned electrolyte membrane as the electrolyte membrane of the PEFC, especially the electrolyte membrane of the DMFC in which a methanol-containing solution is supplied to the fuel electrode.